Internal combustion engines may generate considerable amounts of nitrogen oxides (NOx) during operation. In the case of diesel and gasoline lean-burn engines used in motor vehicles, the amounts of nitrogen oxide in the exhaust gas are generally higher than the admissible limit values, such that exhaust-gas aftertreatment is desired in order to reduce the NOx emissions. In one prior art method, an SCR (Selective Catalytic Reduction) catalytic converter is used for reducing the NOx emissions. For the conversion of the nitrogen oxides by an SCR catalytic converter into non-hazardous substances, a supply of ammonia (NH3) may be needed, for example by way of an injection of a urea solution into the exhaust-gas stream. In another widely used method, use is made of a NOx storage catalytic converter (Lean NOx Trap, LNT) which absorbs and stores the nitrogen oxides contained in the exhaust gas of the internal combustion engine. A regeneration of the NOx storage catalytic converter must be performed from time to time, for which purpose it is necessary for an excess of fuel to be generated in the exhaust gas that is conducted through the NOx storage catalytic converter, which entails an additional consumption of fuel. Thus, both for the operation of an SCR catalytic converter and also of a NOx storage catalytic converter, additional costs are incurred owing to the needed provision of ammonia and of additional fuel.
U.S. Pat. No. 7,472,545 B2 for discloses an exhaust-gas aftertreatment system of an internal combustion engine including a NOx storage catalytic converter which is followed by an SCR catalytic converter. If fuel reformate is supplied to the NOx storage catalytic converter for regeneration purposes, the exhaust-gas stream is conducted past the NOx storage catalytic converter, via a bypass branch that is connected in parallel with respect to the latter, directly to the SCR catalytic converter. However, the inventors have recognized several drawbacks with the exhaust gas aftertreatment system disclosed in U.S. Pat. No. 7,472,545. For instance, during regeneration of the NOx storage catalytic converter engine emissions are increased. Moreover, regeneration of the NOx storage catalytic converter engine may only be implemented during engine operating conditions when the engine is operating at a desirable speed and/or load, thereby limiting regeneration of the NOx storage catalytic converter
As such in one approach, a method for operating an exhaust-gas aftertreatment system arrangement is provided. The method includes in a first operating state, flowing a majority of an exhaust-gas stream from the internal combustion engine through a first NOx storage catalytic converter positioned in a main exhaust branch of an exhaust aftertreatment system arrangement and in a second operating state, flowing a majority of the exhaust-gas stream through a first bypass branch branching off from the main exhaust branch upstream of the first NOx storage catalytic converter and which opens into the main exhaust branch downstream of the first NOx storage catalytic converter and upstream of a second NOx storage catalytic converter and regenerating the first NOx storage catalytic converter.
In this way, the first NOx storage catalytic converter may be regenerated while exhaust gas is directed around the NOx storage catalytic converter and then flowed into the second NOx storage catalytic converter, to reduce emissions during regeneration of the first NOx storage catalytic converter. Therefore, the first NOx storage catalytic converter may be regenerated independently of the engine speed or engine load, if desired. As a result, the NOx storage catalytic converter can be regenerated over a wider time frame of engine operation when compared with prior exhaust systems.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Additionally, the above issues have been recognized by the inventors herein, and are not admitted to be known.